When the same subject is shot by a plurality of cameras, the times may not be synchronized between the videos shot by the cameras. As a method for synchronizing videos shot by a plurality of cameras for which shooting timings are not synchronized, there is employed a method for synchronizing shooting timings of cameras with reference to a modulation timing of an illuminated light by use of an intensity-modulated illumination device (see Patent Literature 1, for example). The method described in Patent Literature 1 is such that an intensity-modulated illumination light is illuminated and a change in luminance of the illumination light is read by a camera so that a temporal shift between the operations of the illumination device and the camera (shooting device) is computed. Then, a frame time of the camera (shooting device) is adjusted based on the computed temporal shift, thereby reducing a temporal shift.
FIG. 10 is an explanatory diagram illustrating an exemplary structure of a synchronization device. The synchronization device illustrated in FIG. 10 is a device to which the method described in Patent Literature 1 is applied. As illustrated in FIG. 10, the synchronization device includes temporal function generation means 1, shooting means 2, feature computation means 3, temporal correlation computation means 4, and illumination means 5.
The shooting means 2 shoots videos of a subject 8, and outputs the shot videos to the feature computation means 3. The feature computation means 3 has an input of the videos output from the shooting means 2 and outputs a feature temporal function b(t) to the temporal correlation computation means 4. The temporal correlation computation means 4 compares a temporal function a(t) output from the temporal function generation means 1 with the feature temporal function b(t) output from the feature computation means 3 thereby to generate and output time synchronization information.
The operations of the synchronization device illustrated in FIG. 10 will be described below.
At first, the temporal function generation means 1 outputs the temporal function a(t) taking a value defined per frame. Specifically, a rectangular waveform function having a duty ratio of 1:1 at which a positive or negative value is taken for each frame is employed. The temporal function a(t) is output to the illumination means 5 and the temporal correlation computation means 4.
The illumination means 5 illuminates an intensity-changed light on the subject 8 depending on the value of the input temporal function a(t).
The shooting means 2 shoots the subject 8 illuminated with the light from the illumination means 5, and acquires and outputs videos of the subject to the feature computation means 3. The feature computation means 3 computes a total of luminance values in a frame as the feature from the video of the subject, and computes the feature temporal function b(t) as a function indicating a temporal change in the computed feature. The computed feature temporal function b(t) is output to the temporal correlation computation means 4.
The temporal correlation computation means 4 computes a temporal correlation between the temporal function a(t) and the feature temporal function b(t) in the following Equation (1).
                              [                      Mathematical            ⁢                                                  ⁢            Formula            ⁢                                                  ⁢            1                    ]                ⁢                                                                                      q        =                              1            T                    ⁢                      ∫                                          a                ⁡                                  (                  t                  )                                            ⁢                              b                ⁡                                  (                  t                  )                                            ⁢                              ⅆ                t                                                                        Equation        ⁢                                  ⁢                  (          1          )                    
The temporal correlation value q in Equation (1) changes due to a difference in phase between the temporal function a(t) and the feature temporal function b(t). That is, the temporal correlation value q corresponds to a temporal shift between the operations of the illumination means 5 and the shooting means 2. Thus, time synchronization information can be acquired from the temporal correlation value q.